Free piston engine compressor



Jan. 21, 1936. A c E 2,028,331-

FREE PISTON ENGINE COMPRESSOR Filed May 15, 1954 2 Sheets-Sheet 1 Fig.1

F A *n In venton:

Jan. 21, 1936. JANlCKE 2,028,331

FREE PISTON ENGINE COMPRESSOR Filed May 15, 1934 2 Sheets-Sheet 2 Fig.4

In venfor:

Patented Jan. 21, 1936 UNITED STA ES PATENT OFFICE FREE PISTON ENGINE COMPRESSOR many Application May 15, 1934, Serial No. 725,748 In Germany May 20, 1933 13 Claims.

My invention relates to free piston engines in which a. free piston or a pair of such pistons is or are arranged in a cylinder for reciprocation, and more particularly to that kind of free piston engines which comprise a multistage compressor. In the hitherto known engines of this latter kind the compression chambers of the single stages are arranged in a row in alignment with the motor cylinder of the engine.' This construction requires a considerable overall length because each stage increases the length of the engine by an amount corresponding 'to the maximum length of stroke of'the free pistons. This drawback is particularly felt in free piston engines composed of two symmetrically designed halves, in which a pair of free pistons is provided which reciprocate simultaneously in opposite directions. For instance in a symmetrically constructed free piston engine comprising on each side a threestage compressor and a coupling gear connected to crossmembers for coupling the free pistons of the engine with each other, the overall length amounts to the tenfold stroke of the pistons, because'each of the six compressor pistons, the two motor pistons and the two crossmembers requires a portion of the total l'ngth corresponding to the maximum stroke.

It is an object of my invention to improve engines of the kind referred to in such manner that the overall length ofthe engine is considerably reduced. This is accomplished by arranging at least some of the cylinders of the multistage compressor in such manner that one cylinder is arranged within another one so that the overall length of the multistage compressor does not substantially exceed the length of a single-stage compressor operating with a like stroke. Preferably the dimensions are chosen in such manner that in the inner dead centre position the piston of the outer stage closely approaches the outer end of the motor cylinder. Since the diameters of the cylinders and of the pistons operating in the inner cylinders are relatively small, the inner cylinders should be the high pressure cylinders, the outer ones the low pressure cylinders of the compressor. In a preferred embodiment of my invention the diameters of the inner cylinder piston are so small that the inner cylinder can be arranged in the hollow piston rod connecting the motor piston with the piston of the outer compressor cylinder. In this case the piston of the high pressure cylinder may be stationary, the hollow piston rod forming a reciprocating cylinder of theinnerstage. If desired,

however, the diameters of the high pressure cylinder may be chosen so small that a stationary cylinder may be providedfor the high pressure stage and may be arranged within the hollow piston rod.

In engines according to my invention comprising free pistons reciprocating in opposite directions of pistons may be coupled with each other by means of a well known coupling gear, which may be arranged withinthe outer lowpressure cylinder, so that it may be directly connected to the pistons of the low pressure cylinder or to a member connected thereto. By arranging the coupling gear in this manner the dimensions of the engine are still further reduced. 15

In reducing these constructions to practice it becomes necessary to suitably choose the volume and the pressure ratios in the stages. As a rule in a two-stage compressor the total pressure ratio, i. e. the ratio between the discharge pres- 20 sure of the second stage and the suction pressure of the first stage is distributed uniformly over the stages, and similarly the volume ratios, 1. e. the ratio between the volume of the dead space and the stroke volume are made equal in both 25 stages, so that the volumetric efiiciency is the same in both stages also. Now, obviously, in an engine according to my invention the pressure ratios and the volume ratios cannot be chosen in this manner, because, in order to attain this, 30 the diameter of the second stage must be made so large that the second stage cannot be arranged within the hollow piston rod, the outer diameter of which must not exceed the diameter of the motor piston. However, the diameter of the motor piston cannot be chosen at will, because it is determined by the energy to be fed to the compressor cylinders during each stroke, by the stroke itself and by the average specific pressure in the motor cylinder. In consequence 40 thereof in an engine according to my invention the pressure ratios or the volume ratios or both are made to difier from each other in the single stages. For instance, in order to sufficiently reduce the diameter of the second stage, the volume 45 ratio of this stage is reduced as far as possible by reducing the dead space. In consequence thereof the volumetric eificiency is increased and the required stroke volume is decreased so that the diameter is small at a given stroke. The 60 stroke volume of the second stage can be reduced further by providing for a relatively large pressure ratio in the first stage, the gas in the first stage being compressed to a relatively high pressure and a correspondingly small volume during 66 every stroke at which it is delivered to the second stage.

The energy required for returning the pistons from their outer to their inner dead centre positions and for compressing the charge in the motor cylinder (hereinafter called feed-back energy), is delivered in free piston engines substantially by the expansion of the gas bodies remaining over in the dead spaces of the compressor cylinders. Now, if the second stage of the compressor is constructed with a low volume ratio as above described, this stage can feed only a small amount of feed-back energy, and there fore the amount to be fed by the first stage must be correspondingly greater. In consequence thereof the dead space and the volume ratio of the first stage must be relatively large. This requires a very large stroke volume in the first stage in order to suck in the required amount of gas during every stroke. In order to obtain a large stroke volume at a predetermined length of stroke the diameter of the piston of the first stage must be sufliciently large. This is advantageous, since the arrangement of the coupling gear within the cylinder of the low pressure stage is facilitated thereby. In some cases, however, in order to arrange the coupling gear within the cylinder of the low pressure stage, it is necessary to increase the diameter of this cylinder in excess of the value required for obtaining a large volume ratio, so that the eflicient stroke volume and the energy demand of the compressor would be increased to an undesirable extent. In order to overcome this difliculty, I reduce the efficient length of the piston stroke by discharging from this cylinder, during the I initial portion of the working stroke, aportion of the sucked gas. If the compressor operates with air, the discharged air may be used as scavenging air in the motor cylinder.

It should be understood that my invention may be used as well in connection with symmetrically designed free piston engines as with non-symmetrical engines comprising a multistage compressor.

In the drawings aflixed to this specification and forming part thereof some diagrams designed to illustrate the operation of an engine according to my invention and an embodiment of such an engine are shown diagrammatically by way of example.

In the drawings:

Figs. 1 to 3 are pressure volume diagrams, in which the abscissa: designate the volume v and the ordinates the pressure p of the gas.

Fig. 4 is an axial section of a symmetrically designed free piston engine comprising two motor pistons.

Referring first to Figs. 1 to 3, the influence of the pressure ratios and the volume ratios on the sucked-in gas volume and the amount of feedback energy shall be explained. Fig. 1 shows the diagram of an ordinary compressor stage. During the working stroke, beginning at the point B, the gas is compressed along the curve B--C from the pressure 111 to the pressure p2. During the last portion of the working stroke the compressed gas is discharged, this portion being indicated by the horizontal line C--E. The energy fed to the compressor stage during the working stroke is rendered in the diagram by the area of the surface A--B-CE-FA. At the end energy fed to the compressor during the working stroke. During the initial portion of the back stroke this gas body is expanded to the initial pressure in along the curve EG. Thereafter a new gas body is sucked in, this portion of the operating cycle being indicated by the horizontal line GB. The energy fed to the free pistons during the back stroke (the so-called feed-back energy) is rendered in the diagram by the area of the surface A-B-G-E-F-A.

The influence of a variation of the volume ratio to the feed-back energy may be guessed from Fig. 2. The volume ratio is expressed by the formula in which inis the volume of the dead space and on the stroke volume. If the dead space is increased to the value v-r-l-v'r and the volume ratio is correspondingly changed to U r+ r V r-i' r'i- II the diagram is changed to the operating cycle BC'EG'B. In consequence thereof the energy fed to the compressor during the working stroke is diminished by a value indicated in Fig. 2 by the area of the surface B-CC'B, while the feedback energy is increased by an amount corresponding to the area of the surface EGG'E. The length of the suction stroke is decreased to G'B, which may be compensated for by increasing the diameter of the piston.

Referring now to Fig. 3, the influence of a variation of the pressure ratio shall be discussed. If the pressure ratio is increased from of a cylinder, may be varied to a considerable extent, so that one or a plurality of high pressure cylinders may be arranged within the hollow piston rod connecting the motor piston to the piston of a low pressure cylinder. Besides this the diameter of the low pressure cylinder may be made so large that the coupling gear can be arranged within the low pressure cylinder and directly connected to the free pistons, for instance of the low pressure cylinders. It can further be seen from the diagrams that by suitably choosing the pressure ratios and the volume ratios the feed-back energy delivered by the single stages may be varied and chosen in correspondence with the required amount.

In Fig. 4 I is a motor cylinder formed with scavenging ports 4 and exhaust ports 5. 2 and 3 are free pistons arranged for reciprocation in opposite directions within the'cylinder l, the pistons being shown in Fig. 4 in their outer dead centre position. The ports 4 and 5 are controlled 4 by the pistons 2 and ,3. respectively. Each piston is connected with a two-stage compressor by means of a hollow piston rod 8 having, in the embodiment shown in Fig. 4, the same diameter as the pistons. Each piston rod 6 is connected to the piston I. of the associated low pressure compressor cylinder, each piston 'I being ar ranged for'reciprocation in a cylinder 8. The cylinders 8 are made integral with a cylindrical casing enclosing the motor cylinder I. The lengths of the cylinder I and of the piston rods 6 are so chosen that in their inner dead centre positions the pistons I closely approach the ends of the motor cylinder I. Each cylinder 8 is closed at its outer end by means of an end wall I 0 provided with suction valves it through which fresh air is sucked into the cylinders 8 during the return strokes of the pistons. The high pressure stage of each compressor comprises a cylinder II rigidly secured to the associated end wall I 0 and a. piston I2 connected to the associated motor piston by means of a piston rod I3. If desired, the piston rods I3 may be connected to the pistons 2, 3 and I2 by means of links or the like, inorder to avoid jamming of the pistons. The diameters of the cylinders II and of the pistons I2 are smaller than the inner diameters of the hollow piston rods 6, so that each high pressure stage may be arranged within its associated low pressure stage and the cylinders II protrude into the hollow piston rods 6, the overall length.

of the engine being not substantially greater than in an engine comprising single-stage compressors. It should be understood that the pressure ratios and the volume ratios in the compressor stages are to be chosen in the manner discussed above.

I6 are exhaust valves arranged in the walls of the cylinders 8 near the end walls I 0 through which the air compressed in the low pressure stages is discharged into an intermediate chamber I8, from which the air is sucked into the cylinders II of the high pressure stages through suction valves I5 provided in the mid portion of the end walls III. The intermediate chamber I8 may be designed as a cooler for the air compressed in the low pressure cylinder. haust valves arranged in the mid portion of the end walls III through which the air compressed in the high pressure cylinders is discharged into pipes I9 leading the compressed air to a consumer (not shown). 20 are spring-loaded non-return valves for adjusting the pressures in the engine independently of the pressures outside of the engine, so that the required operating pressures'are always maintained in the engine.

In each half of the engine the motor'piston and the compressor pistons connected with each other form a freely movable mass operating as a unit. These units are coupled with each other by means of a coupling gear comprising toothed racks 2 I, 22 and a pinion 23 in engagement therewith. The racks are connected to the pistons I, the pinion 23 being rotatably mounted in the casing of the engine. The diameters of the cylinders 8 and of the casing, with which they are integral, are made so large that the coupling gear and its connections to the pistons I can easily be arranged within the cylinders and the casing. To this end the diameters of the cylinders 8 are greater than would be necessary in order to obtain the required feeding output of the lower II are exwalls of the cylinders 8, so that during the initial portion of the working stroke of the pistons air is discharged from the cylinders 8, until the pistons I pass the outlet valves 24. In the embodiment shown in Fig. 4, which is operated with air, the air discharged from the left hand cylinder 8 during the initial portion of the operation is used as scavenging air by connecting the exhaust valves 24 by means of the connection 25 to the scavenging air chamber 28 surrounding the motor cylinder I andconnected to its interior by means of the scavenging port I. It should be understood that the right end cylinder 8 may, if desired, be connected to the chamber 28 in a similar manner.

' by the space'enclosed between the left piston I and the left end wall 21 of chamber 26. 28 are suction valves arranged in the circumferential wall of the pump space and 29 are pressure valves arranged in t e wall 21, so that air is sucked through valves 28 during the working stroke of the pistons and is pumped into the chamber 26 through valves 29 during the return stroke of the pistons.

In'order to provide a load balance between the racks connected 'to the same piston, for instance between the racks 2 I, a rigid connection between the racks and the associated piston is provided merely at the right-hand piston shown in Fig. 4. In the left-hand compressor, however, the racks 2I are connected to the associated piston 'I by means of an annular cross-member 34 shaped as a spherical segmentand provided with spherical surfaces 30 and 3|, which abut against correspondingly shaped counter-surfaces provided on the piston 1 and a counter member 32 bolted to the piston 1 by means of screw bolts 33. The

cross-member 34 is provided with suitably dimensioned bores 35, through which extend the screw bolts 33, so that the cross-member 34 is movable with respect to the piston I to the extent of securing a load balance between the racks 2i pivoted to the cross-member at 36. T'he'racks 22 are rigidly secured tothe aszociated piston I, because otherwise the coupling gear could not operate. 31 is a fuel inlet formed in the wall of the cylinder I.

The operation of the engine shown in Fig.4 is substantially the same as that of known free piston engines. 'Fuel mixture is periodically fed into cylinder I and burnt therein so that the motor pistons 2 and 3 execute working strokes, during which the air pre 'ent in the compression chambers is compressed and partly delivered to the consumers. The pistons are returned to their inner dead centre positions'by the energy stored in the gas or air bodies remaining over in the dead spaces of the compression chambers, scavenging being effected in the manner described above. It should however be understood that the engine according to the invention does not require as large a space as other free piston engines comprising multistage compressors and that more particularly the overall length of the engine is considerably reduced.

' I wish it to be understood that I do not desire to be limited to the exact details of construction I shown and described vfor obvious modifications will occur to a. person skilled in the art.

Iclaim: 1 v

1. In a free piston motor-compressor in com-. bination,'a motor cylinder, 9. free piston arranged in said cylinder, and amultistage compressor comprising a low pressure cylinder, a

high pressure cylinder and pistons arranged for reciprocation in said two cylinders, said high pressure cylinder being surrounded by said low pressure cylinder. I

2. In a free piston motor-compressor in combination, a motor cylinder, a free piston arranged in said cylinder, and a multistage compressor comprising a low pressure cylinder, a high pressure cylinder and pistons arranged for reciprocation in said two cylinders, and a hollow piston rod connecting said motor piston to the piston of said low pressure cylinder and surrounding said high pressure cylinder.

3. In a free piston motor-compressor, in combination, a motor cylinder, a free piston arranged in said cylinder, and a multistage compressor comprising a low pressure cylinder, a high pressure cylinder and pistons arranged for reciprocation in said two cylinders, and a hollow piston rod connecting said motor piston to the piston of said low pressure cylinder and surrounding said high pressure cylinder, the lengths'of said cylinders and of said piston rod being such that the piston of said low pressure cylinder in its inner dead centre position closely approaches the outer end of said motor cylinder.

4. In a free piston motor-compressor in combination, a motor-cylinder, a free piston arranged in said cylinder,'and a multistage compressor comprising a low pressure cylinder, a high pressure cylinder and pistons arranged for reciprocation in said two cylinders, an end wall forming part of said low pressure cylinder, and a hollow piston rod connecting said motor piston to the piston of said low pressure cylinder, said high pressure cylinder being secured to said end wall of the low pressure cylinder and protruding into said hollow piston rod. 1

5. A free piston motor-compressor comprising in combination, a motor cylinder, a pair of free motor pistons arranged in said cylinder to form a combustion chamber between them, a pair of multistage compressors, each comprising a low pressure cylinder, 3, high pressure cylinder, and free pistons arranged in said cylinders, means for driving said compressor pistons by means of said motor pistons, a coupling gear connecting the pistons of said low pressure cylinders with each other, and ,a cylinder, the outer ends of which are formed by said low pressure cylinders, said cylinder surrounding said motor cylinder, said high pressure cylinders and said coupling gear.

6. A free piston motor-compressor comprising in combination, an outer cylinder, two multistage compressors forming the end portions of said outer cylinder, each compressor comprising a low pressure cylinder and a high pressure cylinder, a motorcylinder arranged in the mid portion of said outer cylinder, a pair of free motor pistons arranged in said motor cylinder, enlarged pistons arranged in the end portions of said outer cylinder and forming part of said low pressure cylinders, hollow piston rods connecting said motor pistons with said enlarged pistons, the high pressure cylinders extending into said hollow piston rods,and a coupling gear in said outer cylinder connecting said enlarged pistons with each other.

7. A free piston motor-compressor comprising in combination, an outer cylinder, two multistage compressors forming the end portions of said outer cylinder, each compressor comprising a low pressure cylinder and a high pressure cylinder, a motor cylinder arranged in the mid portion of said outer cylinder, a pair of free motor pistons arranged in said motor cylinder, enlarged pistons arranged in the end portions of said outer cylinder and forming part of said low pressure cylinders, hollow piston rods connecting said motor pistons with said enlarged pistons, the high pressure cylinders extending into said hollow piston rods, and a coupling gear in said outer cylinder connected rigidly to one of said enlarged pistons and movably to the other enlarged piston.

8. A free piston motor-compressor comprising in combination, an outer cylinder, two multistage compressors forming the end portions of said outer cylinder, each compressor comprising a low pressure cylinder and a high pressure cylinder, a motor cylinder arranged in the mid portion of said outer cylinder, a pair of free motor pistons arranged in said motor cylinder, enlarged pistons arranged in the end portions of said outer cylinder and forming part of said low pressure cylinders, hollow piston rods connecting said motor pistons with said enlarged pistons, the high pressure cylinders extending into said hollow piston rods, a coupling gear in said outer cylinder, a cross-member having the form of a spherical segment arranged to abut against a correspondingly shaped portion of one of said enlarged pistons, said couplinggear being rigidly secured to the other enlarged piston and being pivoted to said cross-member.

9. In a free piston motor-compressor in combination a multistage compressor comprising a low pressure cylinder and a high pressure cylinder arranged within said low pressure cylinder and means associated with at least one of said cylinders for discharging, during the initial portion of the compressing stroke, a portion of the gas contained in the said cylinder.

10. A free piston motor-compressor comprising in combination, a motor cylinder, a free motor piston arranged in said cylinder, a multistage compressor comprising a low pressure cylinder, a high pressure cylinder, and a free piston in each cylinder, a hollow piston rod connecting said motor piston to the piston of said low pressure cylinder and surrounding said high pressure cylinder and an exhaust valve arranged in the wall of said low pressure cylinder so as to discharge, during the initial portion of the outward stroke of said pistons, a portion of the gas contained in said low pressure cylinder.

11. A free piston motor-compressor comprifing' in combination, a motor cylinder, a tree motor piston arranged in said cylinder, a multistage compressor comprising a low pressure cylinder, a high pressure cylinder, and a tree piston arranged in each cylinder, a hollow piston rod connecting said motor piston to the piston of said low pressure cylinder and surrounding said high pressure cylinder, a scavenging air container and an exhaust valve arranged in the wall of said low pressure cylinder so as to discharge during the initial portion of the outward stroke of said pistons, a portion of the gas contained in said low pressure cylinder into said container.

12. In a free piston motor-compressor in combination, a multistage compressor comprising a low pressure cylinder and a high pressure cylinder arranged within said low pressure cylinder means for maintaining, predetermined gas volume and pressure ratios in said cylinders and means inserted behind the last compressor stage in the path of the gas to be conveyed by the compressor, which is adapted tomaintain the compression end pressure of this gas on a predetermined value under all conditions of operation.

13. A free piston motor-compressor comprising in combination, a motor cylinder, at free motor piston arranged in said cylinder, a. multistage compressor comprising a low pressure cylinder, a

5 high pressure cylinder and a free piston arranged in each cylinder, a hollow piston rod connecting said motor piston to the piston 01' said low pressure cylinder and surrounding said high pressure cylinder and means for maintaining predetermined gas volume and pressure ratios in said cylinders.

HERMANN JANICKE. 

